Furnaces for the thermal cracking of hydrocarbons, for example naphtha, are known and generally comprise a burner-heated radiation zone within the furnace chamber.
The hydrocarbons to be cracked are conducted through the radiation zone in one or a plurality of parallel tubular elements.
The reaction kinetics show that the cracking effect is greatly dependent upon the temperature to which the hydrocarbons are heated, and the residence time of the hydrocarbons in the radiation zone. For olefin formation, it is, for example, essential that the reaction energy be delivered at the highest possible rate to the hydrocarbons, meaning that the residence time of the hydrocarbons in the radiation zone should be minimized.
The short residence times ensures a suppression of undesired side reactions and allows maximum temperatures to be used. In general, therefore, it is desirable to provide conditions which combine maximum temperature, minimum resonance time and maximum yield or conversion to obtain the highest reaction efficiency.
Since a given permissible pressure drop and a maximum permissible tube-wall temperature cannot be exceeded, from the fact that the heating surface is proportional to the tube diameter and the throughput is proportional to the square of the diameter, the conclusion that for the brief residence time, small tubes of small throughput should be used.
In conventional furnaces for this purpose, it is not uncommon to use tubes with an internal diameter of about 25 mm and lengths of about 10 m. Such furnaces can be of the type described in German printed application DE-AS No. 18 09 177.
A further increase in the yield over that which has been obtainable with such furnaces has posed problems since an increase in the temperature or a further reduction in the residence time requires use of tubing with smaller bores, i.e. small caliber tubing. Smaller caliber tubing, however, at operating temperatures of the order of 1100.degree. C. does not have the required stiffness and in addition creates a problem of blockage by carbon deposits within the tubing, i.e. as a result of the coking effect.